This invention concerns power transmission devices in which a parameter of a drive element is automatically adjusted responsive to changes in torque transmitted and, more particularly, devices in which the parameter is the effective diameter of a sheave engageable by an endless power transmitting member such as a V-belt.
For convenience, it will be assumed in the following discussion that the input member of the power transmission device is the driven V-belt sheave of a variable speed V-belt drive of the type commonly used, for example, in harvesting machines in driving drop processing elements such as threshing cylinders or in the propulsion of the harvester. Typically, a hydraulic actuator is used to vary the spacing of the halves of a split driver sheave so as to vary effective diameter. The halves of the driven sheave are biased together so as to maintain suitable belt tension at an effective diameter reflecting the controlled adjustment of the drive sheave.
In variable speed V-belt drives, it is well known to make the driven sheave torque responsive (or torque sensing, as it is often called)--see for example U.S. Pat. Nos. 3,881,370 Vogelaar, 4,138,837 Love, and 4,348,197 Oliver. In a torque-responsive sheave, the biasing of the sheave halves together to maintain belt tension is usually a combination of passive spring loading and a camming action in the saxe direction resulting from relative rotational deflection between the two sheave halves under load.
The relative rotational deflection or movement between the sheave halves has always has an adverse effect on belt life and although this, broadly speaking, has been tolerated as a price to pay for the variable speed feature, the trend to bigger capacity mobile harvester machines has led to a demand for variable speed V-belt drives capable of transmitting high horsepower at moderate speeds and within the space constraints typical of mobile machines. Recent developments in V-belt construction have made possible some dramatic increases in drive power capacity, at least in standard fixed speed ratio drives. But, when these improved belt constructions have been used in variable speed drives with the conventional torque-responsive sheave depending on relative rotation between sheave halves, belt like has been disappointingly short. Features of the belt construction which help produce the higher power capacity in a straightforward drive make the belt relatively more susceptible to failure from the asymmetrical forces induced in it by the conventional torque-responsive sheave.
Torque-responsive sheave arrangements which do not depend on relative rotational movement between sheave halves would potentially improve belt life. It is already known to transduce electrically a sensed torque from somewhere in the drive train to control the sheave halves, perhaps through an hydraulic actuator while the sheave halves are held rotationally fixed, one to another. But such methods, at an adequate reliability level, are often not cost effective, especially in the competitive mobile harvester market.